Pipettes and automatic aspirators are constructed preferably with disposable tips or containers, means for mounting the tips or containers, and means for pressurizing or evacuating the contents of the tips or containers. The pressurizing means then comprise a piston chamber, a piston, and means for reciprocating the piston. Most commonly, the piston uses sealing rings that rub against the inside wall of the piston chamber or outside wall of the piston.
Because such rings are subject to wear, bellows have been suggested as an alternative since they have a much longer wear capability. Such bellows can be prestressed and simply compressed manually, as shown in U.S. Pat. No. 4,296,071, or they can be driven by a motor as shown in U.S. Pat. No. 4,041,995, FIGS. 6 and 8. In either case, some manufacturing difficulties are encountered. In the case of a simple bellows that is of the type shown in the '995 patent, it is difficult to attach the base portion of the bellows to the bottom of the piston chamber, simply because of inaccessibility. A solution would be to assemble the chamber housing in several parts, but it is preferred, e.g., for convenience, to have such a housing be one integral part.
One approach to this problem has been to use a flat elastomoeric membrane in place of a bellows, for example, as shown in U.S. Pat. No. 4,444,062, since such a membrane can be secured to the top of the piston chamber. As the piston is advanced, FIG. 4 of the '062 patent, the membrane (68) is stretched into a cylindrical shape. However, such a design encounters the following disadvantages: the elastomeric membrane tends to crack under the high stress that is delivered, and is therefore short-lived. Furthermore, it provides a non-linear displacement since the cross-sectional shape changes as the piston is advanced. A bellows does not suffer these disadvantages. Thus, such a flat membrane is not a suitable substitute for a bellows. Therefore, prior to this invention there has been a need to provide a more readily manufacturable bellows design as a piston ring replacement.
Yet another problem exists with such bellows construction. Many pipettes have a disposable tip ejector comprising a rod extending from the tip mounting surface, back into the end of the piston chamber nearest to the disposable tip. Such rods need to be pushed at the appropriate moment, such as by extending the piston until it contacts the rod. However, when a bellows is disposed in between the piston and the end of the chamber at which the rod is located, excessive power is required to activate the rod. That is, not only must the piston push against the resistance of the rod and tip, but also it must collapse the bellows.
For the above reasons, bellows have had limited utility in use as replacement of piston seals, in spite of their advantage in wearing.